Recent immunocytochemical and histofluorescent studies have established the existence of serotonergic fibers arising from the raphe complex and projecting to the cerebellar cortex. Ultrastructurally, the indoleaminergic fibers are known to establish synaptic contacts as well as diffuse, meandering nonsynaptic terminations on various cell types, including Purkinje cells. These anatomical findings suggest that serotonin may function as a neurotransmitter within the cerebellum, a suggestion that needs to be examined electrophysiologically at the cellular level. The objectives of the proposed research are (1) to define the changes in spontaneous activity of Purkinje neurons induced by exogenously administered serotonin, (2) to pharmacologically characterize the serotonergic nature of the observed responses, and (3) to determine whether the effects of serotonin invoke pre-and/or postsynaptic sites of action. Methods are described for (1) the microiontophoretic application of substances in a manner to determine dose response relationships, (2) single unit recording of the Purkinje cell and (3) the systematic examination of possible presynaptic sites of serotonin effects that include all known afferent putative transmitters to the Purkinje cell.